Effect Of Hydrogen/Halogen Substitution On Structural Phase Transitions Of Organic-inorganic Hybrid Rare-earth Double Perovskite Materials

Organic-inorganic hybrid perovskite materials,which combine the structural characteristics of perovskite materials and the functional characteristics of hybrid materials,can achieve excellent optical,electrical and magnetic properties.It is one of the hot research directions in the field of materials.Among these functional properties,some properties,such as ferroelectricity and ferroelasticity,are closely related to their structural phase transition behavior,tune the phase transition properties is the key to realize the related properties.On the premise of keeping the perovskite structure,fine tuning the structure of organic cations is an important means to adjust the phase transition properties（symmetry,phase transition temperature,etc.）of hybrid perovskite materials.In this thesis,taking advantage of the high tolerance of hybrid rare-earth double perovskites to organic cations,I studied the effect of hydrogen/halogen substitution of organic cations on symmetry and phase transition temperatures on the premise of maintaining the basic structures of organic cations and perovskite.In order to construct perovskite materials with high phase transition temperature,nine organic-inorganic hybrid rare earth double perovskites were synthesized by cationic halogen substitution method:（TMA）₂Rb Eu（NO₃）₆（1）（TMA=tetramethylammonium）,（TMFM）₂Rb Eu（NO₃）₆（2）（TMFM=trimethylfluoromethylammonium）,（TMCM）₂Rb Eu（NO₃）₆（3）（TMCM=trimethylchloromethylammonium）,（TMBM）₂Rb Eu（NO₃）₆（4）（TMBM=trimethylbromomethylammonium）,（TMIM）₂Rb Eu（NO₃）₆（5）（TMIM=trimethyliodomethylammonium）,（HQ）₂Rb Eu（NO₃）₆（6）（HQ=quinuclidinium）,（4FHQ）₂Rb Eu（NO₃）₆（7）（4FHQ=4-fluoroquinuclidinnium）,（S3FQ）₄Rb Eu（NO₃）₈（8）（S3FQ=S-3-fluoroquinuclidinnium）,（S3Cl Q）₄Rb Eu（NO₃）₈（9）（S3Cl Q=S-3-chloroquinuclidinnium）.The main findings include as follows:1、After the synthesis of compound 1,halogen atoms（F,Cl,Br,I）are used to replace one of the H atoms in the organic cation TMA respectively to form derivatives of TMA,and finally to give compounds 2 to 5 with similar structures to 1.The results show that the halogen substitution effectively increases the phase transition temperature.The phase transition temperature of compounds 2 to 5 gradually increases,and the phase transition temperature of compound 5 is 119 K higher than that of 1.2、After the synthesis of compound 6,the F atom is used to replace the 4-site H atoms of the quinuclidinium cation,and finally compound 7 with the similar structure to 6 is synthesized.The results show that the phase transition temperature of compound 7 is 169 K higher than that of compound 6,and compound 7 shows high-temperature ferroelasticity.3、Based on compound 6,compounds 8 and 9 were synthesized by replacing the 3-site hydrogen of the quinuclidinium cation with halogen atoms（fluorine and chlorine）,and accordingly,by introducing chirality into the organic cation of quinuclidinium cation.The symmetry of compounds 8 and 9 is reduced,and the phase transition temperatures are raised by183 K and 125 K compared with that compound 6,respectively.These results show that cationic halogen substitution can effectively tune the structure,symmetry and phase transition temperatures of hybrid perovskites.This provides a new idea for the related performance tailoring of perovskite materials;At the same time,it provides a new method for designing novel perovskite materials and regulating the structure of perovskite materials.